A file with associated buffering is called a stream (see Intro(3)) and
is declared to be a pointer to a defined type FILE. The fopen(3C)
function creates certain descriptive data for a stream and returns a pointer
to designate the stream in all further transactions. Normally, there are three
open streams with constant pointers declared in the <stdio.h> header and associated with
the standard open files:

stdin

standard input file

stdout

standard output file

stderr

standard error file

The following symbolic values in <unistd.h> define the file descriptors that will
be associated with the C-language stdin, stdout and stderr when the application
is started:

STDIN_FILENO

Standard input value

0

stdin

STDOUT_FILENO

Standard output value

1

stdout

STDERR_FILENO

Standard error value

2

stderr

The constant NULL designates a null pointer.

The integer-constant EOF is returned upon end-of-file or error by most integer
functions that deal with streams (see the individual descriptions for details).

The integer constant BUFSIZ specifies the size of the buffers used by
the particular implementation.

The integer constant FILENAME_MAX specifies the number of bytes needed to hold
the longest pathname of a file allowed by the implementation. If
the system does not impose a maximum limit, this value is the
recommended size for a buffer intended to hold a file's pathname.

The integer constant FOPEN_MAX specifies the minimum number of files that the
implementation guarantees can be open simultaneously. Note that no more than 255
files may be opened using fopen(), and only file descriptors 0 through
255 can be used in a stream.

The functions and constants mentioned in the entries of section 3S of
this manual are declared in that header and need no further declaration.
The constants and the following “functions” are implemented as macros (redeclaration of
these names is perilous): getc(), getchar(), putc(), putchar(), ferror(3C), feof(3C), clearerr(3C), and
fileno(3C). There are also function versions of getc(), getchar(), putc(), putchar(), ferror(), feof(),
clearerr(), and fileno().

Output streams, with the exception of the standard error stream stderr, are
by default buffered if the output refers to a file and line-buffered
if the output refers to a terminal. The standard error output stream
stderr is by default unbuffered, but use of freopen() (see fopen(3C)) will
cause it to become buffered or line-buffered. When an output stream is
unbuffered, information is queued for writing on the destination file or terminal as
soon as written; when it is buffered, many characters are saved up
and written as a block. When it is line-buffered, each line of
output is queued for writing on the destination terminal as soon as
the line is completed (that is, as soon as a new-line character
is written or terminal input is requested). The setbuf() or setvbuf() functions
(both described on the setbuf(3C) manual page) may be used to change
the stream's buffering strategy.

Interactions of Other FILE-Type C Functions

A single open file description can be accessed both through streams and
through file descriptors. Either a file descriptor or a stream will
be called a handle on the open file description to which it
refers; an open file description may have several handles.

Handles can be created or destroyed by user action without affecting the
underlying open file description. Some of the ways to create them
include fcntl(2), dup(2), fdopen(3C), fileno(3C) and fork(2) (which duplicates existing ones into
new processes). They can be destroyed by at least fclose(3C) and close(2), and
by the exec functions (see exec(2)), which close some file descriptors
and destroy streams.

A file descriptor that is never used in an operation and could
affect the file offset (for example read(2), write(2), or lseek(2)) is not
considered a handle in this discussion, but could give rise to one (as
a consequence of fdopen(), dup(), or fork(), for example). This exception
does include the file descriptor underlying a stream, whether created with
fopen() or fdopen(), as long as it is not used directly by the
application to affect the file offset. (The read() and write() functions
implicitly affect the file offset; lseek() explicitly affects it.)

If two or more handles are used, and any one of them
is a stream, their actions shall be coordinated as described below.
If this is not done, the result is undefined.

A handle that is a stream is considered to be closed when
either an fclose() or freopen(3C) is executed on it (the result of
freopen() is a new stream for this discussion, which cannot be a
handle on the same open file description as its previous value) or
when the process owning that stream terminates the exit(2) or abort(3C). A file
descriptor is closed by close(), _exit() (see exit(2)), or by one of
the exec functions when FD_CLOEXEC is set on that file descriptor.

For a handle to become the active handle, the actions below must
be performed between the last other user of the first handle (the
current active handle) and the first other user of the second handle
(the future active handle). The second handle then becomes the active handle.
All activity by the application affecting the file offset on the first handle
shall be suspended until it again becomes the active handle. (If a
stream function has as an underlying function that affects the file offset,
the stream function will be considered to affect the file offset.
The underlying functions are described below.)

The handles need not be in the same process for these rules
to apply. Note that after a fork(), two handles exist where one
existed before. The application shall assure that, if both handles will
ever be accessed, that they will both be in a state where
the other could become the active handle first. The application shall prepare for
a fork() exactly as if it were a change of active handle.
(If the only action performed by one of the processes is
one of the exec functions or _exit(), the handle is never accessed
in that process.)

For the first handle, the first applicable condition below shall apply. After the actions required below are taken, the handle may be closed if it is still open.

If it is a file descriptor, no action is required.

If the only further action to be performed on any handle to this open file description is to close it, no action need be taken.

If it is a stream that is unbuffered, no action need be taken.

If it is a stream that is line-buffered and the last character written to the stream was a newline (that is, as if a putc('\n') was the most recent operation on that stream), no action need be taken.

If it is a stream that is open for writing or append (but not also open for reading), either an fflush(3C) shall occur or the stream shall be closed.

If the stream is open for reading and it is at the end of the file ( feof(3C) is true), no action need be taken.

If the stream is open with a mode that allows reading and the underlying open file description refers to a device that is capable of seeking, either an fflush() shall occur or the stream shall be closed.

Otherwise, the result is undefined.

For the second handle: if any previous active handle has called a function that explicitly changed the file offset, except as required above for the first handle, the application shall perform an lseek() or an fseek(3C) (as appropriate to the type of the handle) to an appropriate location.

If the active handle ceases to be accessible before the requirements on the first handle above have been met, the state of the open file description becomes undefined. This might occur, for example, during a fork() or an _exit().

The exec functions shall be considered to make inaccessible all streams that are open at the time they are called, independent of what streams or file descriptors may be available to the new process image.

Implementation shall assure that an application, even one consisting of several processes, shall yield correct results (no data is lost or duplicated when writing, all data is written in order, except as requested by seeks) when the rules above are followed, regardless of the sequence of handles used. If the rules above are not followed, the result is unspecified. When these rules are followed, it is implementation defined whether, and under what conditions, all input is seen exactly once.

Use of stdio in Multithreaded Applications

All the stdio functions are safe unless they have the _unlocked suffix.
Each FILE pointer has its own lock to guarantee that only
one thread can access it. In the case that output needs to
be synchronized, the lock for the FILE pointer can be acquired before performing
a series of stdio operations. For example: